get rid of culvert in DEM?

[NguyenMinh_Huyen]

Hi everyone,

I have a 2m DEM and the culvert system is in it, I want to do flow accumulation but the water couldn't find the way because of the road in the culvert system (which has the elevation of 2m above sea level), how can I modify my DEM, to get rid of that road (or to set the elevation of that road to 0m)?
Thanks in advance?

[MarkBoucher]

You have to process the dem. There are tools in Arc Hydro that do this. The tools let you "burn" a stream through the road embankment so the area behind the embankment is not filled during the fill sinks process.

I've posted several Arc Hydro helps in the link below my signature.

[NguyenMinh_Huyen]

You have to process the dem. There are tools in Arc Hydro that do this. The tools let you "burn" a stream through the road embankment so the area behind the embankment is not filled during the fill sinks process.

I've posted several Arc Hydro helps in the link below my signature.

Thanks Mark!

It's quite a lot of information for me as I've never used ArcHydro before..and I really need to my stuff as soon as possible!So is there any other tools? I mean I just want to modify the DEM, to get rid of the road over the river, to make a new DEM without the culvert system, is that possible? I don't know yet if I'm gonna run the flow accumulation after but I need a new DEM without that road for other purposes. 
I'm thinking of making a polygon shape file over the road, add a new "elevation field" for that polygon (the elevation value = elev of road - elev of the river) then convert it to raster then use raster calculator (new DEM = DEM - raster polygon) to make a new DEM...is that correct? it sounds a little bit ..weird..i'm not sure...

[MarkBoucher]

Arc Hydro tools simply convert the polyline to a raster with a value. The value can be very big. You can make the value a little more than you need to "burn" a stream into the embankment. Then, Arc Hydro subtracts the polyline value from the dem. The fill sinks function fills in the stream to allow the flow direction to go downstream.

You can do this without Arc Hydro. Draw a "stream" polyline through the embankment with the ends of the polyline where the low points are in the stream up and down stream of the embankment. Convert the stream to a raster with a high value, high enough the go below the embankment. (You may need to create a field in the polyline table for this value). Then, with with raster math, subtract your stream raster from the dem. After this, you use the resulting dem with the fill function and move on from there like you did before.

Arc Hydro will make the "burned stream" more than one cell wide. You may need to do that, though I'd try doing it with whatever results the polyline to raster conversion give you. Seem like it should work.

[NguyenMinh_Huyen]

Arc Hydro tools simply convert the polyline to a raster with a value. The value can be very big. You can make the value a little more than you need to "burn" a stream into the embankment. Then, Arc Hydro subtracts the polyline value from the dem. The fill sinks function fills in the stream to allow the flow direction to go downstream.

You can do this without Arc Hydro. Draw a "stream" polyline through the embankment with the ends of the polyline where the low points are in the stream up and down stream of the embankment. Convert the stream to a raster with a high value, high enough the go below the embankment. (You may need to create a field in the polyline table for this value). Then, with with raster math, subtract your stream raster from the dem. After this, you use the resulting dem with the fill function and move on from there like you did before.

Arc Hydro will make the "burned stream" more than one cell wide. You may need to do that, though I'd try doing it with whatever results the polyline to raster conversion give you. Seem like it should work.

"Then, with with raster math, subtract your stream raster from the dem." Could you explain to me in more details this step? I tried with raster calculator but I didn't get the right resulting DEM? which tool exactly should I use to do this "subtract"?
Thanks a lot.

[NguyenMinh_Huyen]

Hi Mark,

I also tried to burn a stream over the bridge in ArcHydro.
First I created a shape file of a stream then --> Terrain Processing-->DEM Manipulation-->DEM Reconditioning..then I got the Agree DEM but there's definitely sth wrong with my AgreeDEM, it's just the line...I thought I should have obtained a DEM with a stream line within it?

[MarkBoucher]

then I got the Agree DEM but there's definitely sth wrong with my AgreeDEM, it's just the line...

Nguyen,

Was the result a ployline or a "line" in the DEM? It could be a DEM with an extreme value where the stream was burned and so it may just look like a "line". You can go to the properties>symbology tab and scroll to the bottom. Set the Statistics to be "From current display extent" and then zoom to an area where AgreeDEM should be defined by the stream is out of view. The color ramp should adjust to the view extents and you should see contrasts in the AgreeDEM like the oringinal DEM.

You can also use the identify tool and click in areas away from the "line" where the AgreeDEM should exist and see if it is defined there.

If AgreeDEM is not defined beyond the "line", it could be that your processing extent does not include all of your old DEM and was somehow changed to only include the stream ployline. Certain processes will change the processing extent without warning, don't ask me why.

To make sure processing extent is what it should be, go to menu item Geoprocessing>Environments>Processing Extents and set the processing extents to match your DEM raster. Then try again.

Let us know if this works for you.

[NguyenMinh_Huyen]

hi Mark,

Thanks alot...
it worked perfectly, the small raster now has the same extent as the big one and I can continue with the raster calculator now..I have a lot to do so I haven't managed time to go back to "the line" in the ArcHydro yet but I will, in several days, then I'll get back to you.
Thanks ^^